1) Field of the Invention
The present invention relates to a technology to reduce spreading or spot positioning error of a beam caused by defocusing of the beam.
2) Description of the Related Art
FIG. 13 is a schematic diagram of a generally used scanning image forming optical system. The system includes a laser light source 10, a beam diameter correcting lens 12, an aperture 14, a cylindrical lens 16, a mirror 18, a polygon mirror 20 as a light polarizer, an optical scanning lens 30, and a photosensitive surface 40 as a surface to be scanned.
The luminous flux emitted from the laser light source 10 reaches the polygon mirror 20 through the beam diameter correcting lens 12, the cylindrical lens 16, and the mirror 18. The reflected luminous flux is polarized by the polygon mirror rotating in one direction, enters into the optical scanning lens 30, while moving in one direction, and is scanned in one direction at a substantially constant velocity on the photosensitive surface 40.
As the optical scanning lens 30 used in the scanning image forming optical system, one formed by molding a plastic material is now being used. As one problem when the optical scanning lens 30 is formed by molding a plastic material, there is a fatal problem in that refractive index distribution occurs in the formed optical scanning lens.
In the plastics molding, a thermally melt plastic material is molded by a mold, and cooled in the mold. However, since cooling at the peripheral portion is faster than that at the center of the mold, non-uniform distribution in density occurs in the plastic, that is, a phenomenon in which the density in the portion where cooling occurs quickly becomes relatively higher with respect to the density in the portion where cooling occurs slowly, or transforming occurs, and the refractive index is not uniform inside the formed lens, thereby causing the refractive index distribution.
When there is the refractive index distribution, a change in the focal length, that is, defocus occurs with respect to a focal length designed as a uniform refractive index, and a beam waist position changes from the designed position. Therefore, it is desired, in view of the optical performance, that the refractive index distribution is small. However, it imposes a large restriction on the cost, at the time of machining it.
An example is known, in which a change in a spot diameter on the photosensitive member, which occurs due to a non-uniform change of the beam waist position with respect to the main scanning direction, is suppressed within a tolerance (for example, see Japanese Patent Application Laid-open No. H10-288749).
Generally, the refractive index becomes high at the peripheral portion of the lens, and becomes minimum near the center of the area through which the luminous flux passes. However, it is known that there is an example in which the refractive index becomes the largest in the vicinity of the center, according to the manufacturing conditions. In the explanation below, an example in which the distribution curve is basically protruding downward will be explained.
However, it has been found that in the vicinity of the center, the refractive index distribution is not always stable, and there is a large difference for each molding article. This is because slight additives and impurities added at the time of molding are likely to gather in the vicinity of the center, due to different properties such as density from those of the plastic material, and as a result, a difference in the refractive index distribution occurs, which is slightly different for each molded article.
It is necessary to suppress the occurrence of out-of-color registration, in order to realize a high quality color image, and hence it is necessary to improve the position accuracy of the beam spot. Hence, the difference in refractive index distribution should be suppressed for each scanning lens as much as possible. In order to do that, it is necessary to know how the refractive index distribution of the actual scanning lens is, but fortunately, a nondestructive method of studying the refractive index distribution has been established (for example, see Japanese Patent Application Laid-open No. H11-44641).
On the other hand, it is generally said that the refractive index distribution occurring due to the plastics molding shows distribution close to a quadratic curve, and approximated by the following quadratic expression:
 Δn(x)≈n0+Δn·x2(Δn>0)
However, in a strict sense, there is a tendency such that as it is away from the minimum value, the quadratic coefficient Δn becomes gradually smaller locally. Since the defocus amount of the beam is in proportion to the quadratic coefficient Δn, it is desired that Δn be as small as possible.
Therefore, by designating the area away from the minimum value as an area through which the luminous flux of the beam passes, an area having Δn as small as possible is used. As a result, the influence of defocus due to the refractive index distribution can be reduced.
Recently, image forming apparatus using four photosensitive drums becomes predominant, with a progress of colorization. It is necessary to develop a lens and an optical scanner for achieving high speed, high quality image, low cost, and space saving. With respect to such demands, it is necessary to provide an optical scanning lens in which the refractive index distribution is reduced.
If a difference in the refractive index distribution for each product is reduced, and an optical scanning lens having a stable quality can be obtained, it is advantageous for obtaining a multi-beam. Realization of the multi-beam can reduce the number of rotation of a deflector such as a polygon scanner. As a result, high durability, low noise, and low power consumption can be realized.
FIG. 14 depicts a beam diameter spreading when a defocusing of the beam occurs. The X axis indicates a defocus amount, designating the photosensitive surface as a reference value 0, and the Y axis indicates a diameter of the luminous flux, a so-called beam diameter. If the optical system is manufactured according to the design, the beam diameter is such that, as shown in FIG. 14, the beam waist is located on the photosensitive surface. In other words, the beam diameter becomes minimum on the photosensitive surface, and an image having a high resolution can be obtained. However, as described above, if there is the refractive index distribution in the optical scanning lens, defocus occurs. When the distribution is such that the refractive index becomes minimum in the center portion, the distribution works in the direction that the lens power is reduced, thereby extending the focal length. Therefore, the beam waist moves in the direction away from the optical scanning lens, and as shown in FIG. 14, the beam diameter on the photosensitive surface increases.